1. Field of the Invention
The present invention relates to a manufacturing method of a pattern transfer mask, and more particularly, to a manufacturing method of a pattern transfer mask with an improved usable depth of focus (UDoF) range.
2. Description of the Prior Art
In the continuously improved semiconductor integrated circuit technology, the patterns of the semiconductor devices become finer for increasing the integrity of the integrated circuit. However, the finer the pattern becomes, the smaller the requirement for size uniformity becomes. As the patterns are generally formed by photolithography processes, deviations in the exposure process may be a factor influencing the size uniformity of the patterns.
In the exposure process, many parameters such as the exposure amount and the depth of focus may contribute to the critical dimension (CD) results. The pattern transfer mask used in the exposure process generally has pattern groups with different pattern densities. Based on the acceptable CD variation ranges of the result patterns, each of the pattern groups on the pattern transfer mask has an individual depth of focus (iDoF) range, and a usable depth of focus (UDoF) range of the pattern transfer mask can be defined by the iDoF ranges of the pattern groups with different pattern densities. As the integrated circuit designs become more complicated, the UDoF range of the pattern transfer mask becomes smaller, and the process window will be limited accordingly.